Some integrated circuits (i.e., chips) have an on-chip voltage generator for supplying a voltage different from the voltage supplied to the chip from on off-chip supply. This type of voltage generator is referred to herein as an on-chip supply voltage generator. One typical application for an on-chip supply voltage generator is for generating a negative voltage to back-bias a ptype substrate of a chip to control the threshold voltage of n-channel field effect transistor devices in the chip. FIG. 1 is a simplified functional block diagram illustrative of a conventional back-bias charge pump circuit 10.
Circuit 10 includes a voltage sensing circuit (VSC) 11 and a charge pump 13. VSC 11 is connected to receive the off-chip supply voltage Vdd through a connection 15 and the back-bias voltage Vbb through a connection 16. VSC 11 is configured to generate a control signal Vok, which is received by charge pump 13. More specifically, VSC 11 asserts signal Vok when the value of voltage Vbb reaches a predetermined threshold value, which causes charge pump 13 to stop pumping charge from the substrate through a connection 17.
FIG. 2 is a diagram illustrative of the back-bias voltage generated by back-bias charge pump circuit 10 (FIG. 1). Referring to FIGS. 1 and 2, VSC 11 detects when voltage Vbb generated by charge pump 13 reaches the predetermined value Vmax. When the value of voltage Vbb is below value Vmax (in magnitude in this embodiment because voltage Vbb is maintained at a negative value relative to a ground bus), VSC 11 de-asserts control signal Vok, causing charge pump 13 to operate, thereby increasing the magnitude of voltage Vbb. When the magnitude of voltage Vbb reaches value Vmax, VSC 11 asserts control signal Vok, causing charge pump 13 to stop operating. The value of voltage Vbb then begins to drop. When the value of voltage Vbb drops below the value Vmax, VSC 11 de-asserts control signal Vok, thereby again activating charge pump 13.
This single trip point scheme results in voltage Vbb hovering around voltage Vmax, as indicated by waveform 21 in FIG. 2. It is observed that in this scheme, VSC 11 repeatedly activates and deactivates charge pump 13 with a relatively high frequency. This repeated unnecessary activation and deactivation of charge pump 13 tends to dissipate a relatively large amount of power. In addition, the repeated activation and deactivation of charge pump 13 tends to increase noise in the voltage Vbb. Accordingly, there is a need for a regulator system for on-chip voltage generators with decreased power dissipation and noise generation.